The present invention relates to scroll-type fluid machine, for providing a stable high performance over a wide range of operating conditions.
In, for example, a hermetic scroll type compressor for a refrigeration cycle a scroll-type compression mechanism and a driving motor for driving the mechanism are provided, with the compression mechanism and the motor being hermetically encased by a common casing. The scroll-type compression mechanism includes a stationary scroll member, an orbiting scroll member orbiting with respect to the stationary scroll member, a crankshaft connected to the driving motor for causing the orbiting movement of the orbiting scoll member, and a frame for carrying the stationary and orbiting scroll members as well as the crankshaft. One important requisite for a hermetic scroll compressor is the avoidance of an internal leakage of the fluid under compression. More particularly, as the fluid pressure of the compressor is increased, the axial force produced by the compressed fluid acting to axially separate the stationary and orbiting scroll members is increased to unfavorably increase the tendency of the fluid to leak internally from the high-pressure side to the low-pressure side. To avoid this problem in, for example, U.S. Pat. No. 4,365,941, the fluid of a medium pressure under compression is led to the back side of the end plate of the orbiting scroll member to generate an axial thrust force thereby axially pressing the orbiting scroll member into close contact with the stationary scroll member. In, for example, U.S. Pat. No. 3,884,599 another solution is proposed wherein a high fluid pressure is continuously applied to a back side of the end plate of the orbiting scroll member to maintain the orbting scroll member in close contact with the stationary scroll member. However, these proposals are unsatisfactory since in the arrangement of U.S. Pat. No. 4,365,941, the pressure applied to the back side of the orbiting scroll member is derived from a predetermined portion of the compression chamber formed between the wraps of the orbiting and stationary scroll members and, therefore, is determined solely by the suction pressure of the compressor regardless of, for example, the discharge pressure of the compressor. Therefore, when the discharge pressure of the compressor is increased, the axial separating force tending to axially separate the orbiting and stationary scroll members from each other is increased to overcome the axial force produced by the fluid pressure acting on the back side of the end plate of the orbiting scroll member. Consequently, the gap between the axial end of the wrap of the orbiting scroll member and the end plate of the stationary scroll member is increased to allow the internal leakage of the fluid under compression thereby lowering the volumetric efficiency and seriously impairing the compression performance of the compressor. The increased rate of internal leakage of the fluid increases the leak of the lubricating oil suspended by the fluid, so that the driving torque of the compressor is increased due to an increment of the frictional resistance attributable to shortage of the oil which constitutes the oil film between the ends of the wraps and opposing end plates. Consequently, the load imposed on the driving motor is disadvantageously increased. The lubricating oil is usually supplied through an axially extending oil passage in the crankshaft, with the oil being supplied through the oil passage being discharged to a space formed between the upper end of the crankshaft and a bearing boss provided on the back side of the orbiting scroll member, and the oil then being distributed to portions requiring lubrication such as, for example, an area of contact between the orbiting and stationary scroll members. Therefore, an excessive internal leakage of the lubricating oil may cause an upward shifting of the crankshaft due to a reduction of the oil pressure in the space between the upper end of the crankshaft and the bearing boss. The upward shifting of the crankshaft will bring the end surface thereof for carrying a balance weight into contact with the end surface of the bearing boss, resulting in an increased frictional resistance and, hence, a greater power demand for the driving motor as well as a rapid wear of the contacting surfaces.
On the other hand, in the arrangement of U.S. Pat. No. 3,884,599, the axial force for pressing the orbiting scroll member into closer contact with the stationary scroll member is determined solely by the discharge pressure of the compressor. Therefore, if the pressure in the low-pressure side of the refrigeration cycle is lowered to reduce the suction pressue of the compressor, the internal pressure of the compressor is lowered to decrease the axial separating force acting between the orbiting and stationary scroll members. Consequently, the axial pressing force produced by the fluid pressure acting on the back side of the orbiting scroll member is increased, which, in turn, seemingly increases the resistance caused by the friction between the orbiting and stationary scroll members, requiring a greater input by the driving motor.
To avoid these problems, some proposals of operating the compressor under limited operating pressure and forming lubricating oil grooves in the axial end surfaces of the wraps of both scroll members to enhance the wear resistance and the sealing efficiency have been made such as in, for example, U.S. Pat. No. 3,994,633.
Accordingly, an object of the invention is to provide a scroll-type fluid machine which can stably operate so as to exhibit high performance over a wide range of operating conditions, without requiring any limitation of operating pressure and without requiring any specific anti-friction and sealing construction on the axial end surfaces of the wraps of scroll members.
In accordance with the invention, a scroll-type fluid machine includes an orbiting scroll member and a stationary scroll member each having an end plate and a spiral wrap protruding upright from one of the sides of the end plates, with the orbiting scroll member and said stationary scroll member being assembled together with their wraps meshing with each other such that compression spaces of varying volume are defined by said end plates and said wraps of said orbiting and stationary scroll members. The orbiting scroll member is adapted to make an orbiting movement with respect to the stationary member so that the compression spaces are progressively moved radially inwardly while decreasing their volumes. The said orbiting scroll member having a back pressure chamber formed on the back side thereof and communicating with the compression spaces of decreasing volume through pressure equalizing ports formed in the orbiting scroll member, wherein each of the wraps has at least two turns and, wherein the positions of the pressure equalizing ports in terms of the wrap angle .lambda. of the wraps meet the following condition: EQU .lambda.d&gt;.lambda.&gt;.lambda.d-2.pi.
where, .lambda.d represents a wrap angle of the wraps when the volume of the compression spaces is minimized. The equalizing ports are positioned at .lambda. and .lambda.+2.pi..
According to this arrangement, the pressure introduced into the back pressure chamber on the back side of the orbiting scroll member through the pressure-equalizing ports is affected by the discharge pressure of the compressor and the pressure under the compression. Since the pressure of the fluid under compression is determined by the suctin pressure of the compressor, the axial pressing force for pressing the orbiting scroll member into contact with the stationary scroll member is determined in the scroll-type fluid machine of the invention by the suction pressure and the discharge pressure. Therefore, even if the compression ratio of the compressor is changed due to a change in the suction pressure and/or the discharge pressure, the pressure acting in the back pressure chamber is changed following the change in the internal pressure of the compressor so that the end plate of the orbiting scroll member can be stably pressed at a moderate force which is neither too large nor too small.
It is, therefore, possible to obtain a high performance and stable operation of a scroll-type fluid machine over a wide range of operating conditions, without requiring any limitation of the operating pressure and without requiring specific antifriction or sealing measures on the end surfaces of the wraps of the orbiting and stationary scroll members.
According to the invention, one pressure-equalizing port can take any desired position within the range of .lambda.d&gt;.lambda.&gt;.lambda.d-2.pi., while each pressure-equalizing port is formed at a position near to the wrap of the orbiting scroll member to be of a diameter substantially equal to or smaller than the width of the wrap of the stationary scroll member. As the position of the pessure-equalizing port gets nearer to the position .lambda.d, the back pressure chamber is maintained in communication with the compression space for a longer period of time and, hence, the pressure in the back pressure chamber is more significantly affected by the pressure in the compression space than by the discharge pressure. That is, the mean value of the pressure acting on the rear side of the orbiting scroll member becomes closer to the pressure of the fluid under compression. In contrast, as the position of the pressure-equalizing port gets closer to the position .lambda.d-2.pi., the pressure in the back pressure chamber is affected more significantly by the discharge pressure than in the case where the port takes a position near the position .lambda.d. Consequently, the mean value of the pressing pressue is increased to produce a greater force for urging the orbiting scroll member into contact with the end plate of the orbiting scroll member.
For reducing the frictional resistance, the force for pressing the orbiting scroll member into contact with the stationary scroll member should be decreased. From this point of view, it is preferred that the pressure-equalizing port takes a position near the position .lambda.d.
The above and other objects, features and advantages of the invention will become more apparent from the following description of the preferred embodiment when the same is read in conjunction with the accompanying drawings.